1. Field of the Invention
The present invention relates to a heat fixing apparatus for heat fixing an unfixed toner image corresponding to objective image information that is formed and borne on a recording material (i.e. transferring material, printing paper, photosensitive paper or electrostatic recording paper etc.) as a fixed image, in an image formation process part of an image forming apparatus using an image formation process such as an electrophotography process or an electrostatic recording process etc.
2. Description of Related Art
Conventionally, in fixing apparatus equipped in image forming apparatus using an electrophotography process, an electrostatic recording process or the like, so-called heat fixing apparatus have been widely used. The heat fixing apparatus is adapted to cause a recording material that bears an unfixed toner image to pass through a nip portion (i.e. a fixing nip portion) formed by a fixing roller and a pressure roller, which are rotating while being in pressure contact with each other, to fix the unfixed toner image on the recording material as a permanent image.
1) Heat Fixing Apparatus Using Heating Roller Scheme
An example of the conventional heat fixing apparatus is shown in FIG. 10. This heat fixing apparatus uses a heating roller scheme.
In FIG. 10, reference numeral 40 designates a fixing roller serving as a fixing means (or heating means). The fixing roller includes a halogen lamp 41 housed in a hollow metal core 42 made of aluminum having a thickness of about 0.5 to 4 mm so that it would have a satisfactory mechanical strength. The halogen lamp 41 is supplied with electrical power from a power source (not shown), so that heating sufficient for fusing toner on a recording material P would be performed from the interior of the hollow metal core 42.
In order to fix the toner on the recording material P without offset, a releasing layer 43 made of a material having a good releasability (or releasing property), such as polytetrafluoroethylene (PTFE) or perfluoroalkoxytetrafluoroethylene copolymer etc, is formed on the outer surface of the hollow metal core 42. The releasing layer 43 is formed in a tubular shape or applied by electrostatic spraying or dipping etc.
In some cases, an electro-conductive material such as carbon black is introduced into the releasing layer in order to prevent the offset that might occur in the case that the surface of the fixing roller is charged up upon conveying recording materials.
The hollow metal core 42 of the fixing roller 40 is electrically connected to earth or grounded via a diode element or applied with a bias by a bias applying means (not shown), so that generation of an offset image due to charging-up of the surface of the fixing roller is prevented from occurring.
In addition a thermistor in contact with the surface of the fixing roller 40 is provided. Thus, the temperature of the surface of the fixing roller is detected and the power supply to the halogen lamp 41 is controlled to be turned on and off in such a way that a toner image on a recording material is heated at an appropriate temperature.
On the other hand, reference numeral 50 designates a pressure roller serving as a pressurizing member, which is kept, by a pressing spring (not shown), in pressure contact with the above-mentioned fixing roller 40 at both end portions with respect to the longitudinal direction so as to hold and convey a recording material between them.
The pressure roller 50 is made by providing an elastic layer formed by molding a silicon rubber or a rubber foam elastic layer 52 formed by foaming a silicon rubber on the outer surface of a metal core 51 and further providing, on the outer surface of the elastic layer 52, a releasing layer 53 in the form of a tube or a coating made of PTFE, PFA or FEP etc. in a similar manner as the fixing roller.
With the elasticity of the pressure roller 50, a sufficient width of the nip can be formed between both the rollers 40 and 50. The toner image on the recording material P that is held and conveyed in the nip portion N is fixed by heat applied by the fixing roller 40.
2) Heat Fixing Apparatus Using Film Heating Scheme
Examples of the heat fixing method using a film heating scheme, in which power consumption is reduced as low as possible by not supplying electrical power to a heat fixing apparatus under a stand-by state, are disclosed in Japanese Patent Application Laid-Open No. 63-313182, Japanese Patent Application Laid-Open No. 2-157878, Japanese Patent Application Laid-Open No. 4-44075 and Japanese Patent Application Laid-Open No. 4-204980 etc. Specifically, in the heat fixing method using the film heating scheme, a toner image on a recording material is fixed via a thin film provided between a heater and a pressure roller.
FIG. 11 shows the outline of the structure of an example of the heat fixing apparatus using the film heating scheme. In the structure shown in FIG. 11, a fixing member 60 is mainly composed of a heating member (or a heating body, which will be referred to as a heater hereinafter) fixedly supported on a stay holder (a supporting member) 62 and a thin film (which will be referred to as a fixing film hereinafter) 63 having heat resisting properties loosely fitted over the stay holder 62. In order for a nip portion (i.e. a fixing nip portion) N having a predetermined nip width to be formed, a predetermined pressing force is generated between the fixing member 60 and an elastic pressure roller 50 serving as a pressurizing member by pressurizing means (not shown).
The heater 61 is composed of a ceramic substrate made of alumina and a resistance layer for generating heat with electric power and a protection layer, such as a glass layer or a polyimide layer, that are formed on the ceramic substrate. The heater 61 is heated by power supply to the resistance layer for generating heat with power supply, and the temperature of the heater 61 is controlled to a predetermined temperature by a temperature control system including temperature detection means 64 provided on the back side of the heater 61.
The fixing film 63 is a member in the form of a cylinder, a endless belt or a rolled web having ends that is to be conveyed by driving means (not shown) or by the rotational force of the pressure roller 50 to move in the direction indicated by an arrow while being in close and sliding contact with the surface of the heater 61 in the fixing nip portion N.
A recording material P serving as a material to be heated, on which an unfixed toner image is formed and borne, is introduced into the fixing nip portion N between the fixing film 63 and the pressure roller 50 under the state in which the heater is heated and controlled to a predetermined temperature and the fixing film is conveyed to be moved in the direction indicated by an arrow. Then, the recording material P is held and conveyed in the fixing portion N together with the fixing film 63 while being in close contact with the surface of the fixing film 63. In this fixing nip portion N, the recording material and the toner image are heated by the heater 61 via the fixing film 63, so that the toner image on the recording material P is heated and fixed. A portion of the recording material that have passed through the fixing nip portion N is detached from the surface of the fixing film 63 and conveyed further.
The fixing film 63 is designed to have a considerably small thickness of 20 to 70 μm in order to enable efficient transfer of heat supplied by the heater 61 to the recording material as the material to be heated in the fixing nip portion N. As shown in FIG. 12, the fixing film 63 has a three layer structure including a film base layer 63a, an electro-conductive primer layer 63b and a releasing layer 63c with the film base layer 63a facing the heater side and the releasing layer 63c facing the pressure roller 50 side.
The film base layer 63a is a resin film made of a highly insulative material such as polyimide, polyamide-imide or PEEK etc. or a thin metal film made of SUS or Ni etc. The film base layer 53a has heat resisting properties and a high elasticity with a thickness of about 15 to 60 μm, which ensures flexibility. The film base layer 63a ensures physical strength, such as tearing strength, of the fixing film 63 as a whole.
The electro-conductive primer layer 63b is formed as a thin layer with a thickness of about 2 to 6 μm. In order to prevent charging-up of the fixing film as a whole, the electrically conductive primer layer 63b is either electrically connected to earth or connected to a diode connection or bias applying means.
The releasing layer 63c is a layer for preventing the toner offset onto the fixing film 63. The releasing layer 63c is formed as a coating made of a fluorocarbon resin having a good releasing property such as PFA, PTFE, FEP or the like with a thickness of about 5 to 10 μm. In addition, in order to relieve charging-up of the surface of the fixing film 63 and to prevent the electrostatic offset, an electrically conductive material such as carbon black or the like having a resistivity of about 103 to 106 Ω.cm is added in the releasing layer.
The pressurizing member 50 has a structure similar to the pressure roller 50 of the above-described heat fixing apparatus using the fixing roller scheme.
In the heat fixing apparatus using the film heating scheme as described above, no electrical power is supplied to the heater 61 during a stand-by state, and it is possible to heat-fix an unfixed image on a recording material P by supplying electrical power to the heater to heat itself rapidly up to a temperature that enables fixing during the time from the image forming apparatus's receiving of a print signal to the arrival of a recording material P at the fixing nip portion N. Therefore, the heat fixing apparatus using the film heating scheme is an energy saving heat fixing apparatus.
However, recently the number of the types of recording materials has been increased and there is variety in the thickness, surface quality and resistance etc. of the recording materials. In connection with this, various problems concerning the image have occurred in the heat fixing process performed by a heat fixing apparatus in an image forming process. Those problems have been overcome by various structures.
For example, in the above-described conventional heat fixing apparatus, when a recording material enters the fixing nip portion, a phenomenon that the unfixed toner image on the recording material is scattered in the direction opposite to the recording material conveying direction sometimes occurs (this phenomenon will be referred to as “smeared image trailing edge upon fixing” hereinafter). The mechanism of generation of the smeared image trailing edge upon fixing will be described here with reference to FIG. 13. As shown in FIG. 13, moisture in the recording material is heated in the fixing nip portion N rapidly to evaporate, and toner T of the unfixed toner image on the recording material P in an area that has not entered the fixing nip yet is blown by the blow 80 of the generated vapor in the direction opposite to the recording material conveying direction, so that the smeared image trailing edge upon fixing is generated. This is an image error that is apt to occur under the condition that the moisture content of the recording material is high under high humidity environment, the image pattern includes horizontal lines with a large line width, and the toner amount of the unfixed toner image is large. In addition, it has been found that the degree of the smeared image trailing edge upon fixing has been deteriorated with the speeding-up of the image forming apparatus with which the blow 80 of the vapor generated from the recording material P is intensified.
An attempt to improve the smeared image trailing edge upon fixing will be described in the following. As shown in FIGS. 10 and 11, at a position downstream of the fixing nip portion N formed by the fixing member 40 or 60 and the pressure roller 50 as the pressurizing member with respect to the recording material conveying direction (the upstream side along the conveying direction is defined as the side closer to the starting point of the conveying), there is provided a rubber discharge roller 71 and a discharge roller 72 in a pair that hold and convey the recording material discharged from the fixing nip portion N between them. The rubber discharge roller 71 is made of an electro-conductive rubber material. The rubber discharge roller 71 is electrically grounded. Alternatively, an electro-conductive member in the form of a grounded brush-like member or the like is provided in the downstream of the fixing nip portion N with respect to the recording material conveying direction in such a way that the recording material P is in contact with the electro-conductive member while the recording material P is conveyed. On the other hand, in the above-described conventional apparatus, a bias having the polarity same as that of the unfixed toner image is applied to the hollow metal core 42 of the fixing roller 40 and the electro-conductive primer layer 63b of the fixing film 63 by bias applying means (not shown).
Thus, when the recording material is passing through the fixing nip portion N and in contact with the electro-conductive rubber discharge roller 71, a current path is formed via the recording material P, so that a voltage drop occurs between the fixing roller 40 or the fixing film 63 and the recording material P. This generates an electric field that intensifies retention of the unfixed toner image to the recording material P, and therefore the smeared image trailing edge upon fixing is prevented from occurring.
However, in the case that the speed of the image forming apparatus is increased, the smeared image trailing edge upon fixing is easy to occur, and in order to prevent the smeared image trailing edge upon fixing, it is necessary to increase the voltage drop generated between the fixing roller 40 or the fixing film 63 and the recording material P. Therefore, it is necessary to set a large bias value to be applied to the hollow metal core 42 of the fixing roller 40 or the electro-conductive primer layer 63b of the fixing film 63 and to supply the current path formed through the recording material P with a large current.
However, in the above-described structure in which the current path is formed between the hollow metal core 42 of the fixing roller 40 or the electro-conductive primer layer 63b of the fixing film 63 and the electro-conductive rubber discharge roller 71 via the recording material P, if an excessive current flows in the current path, an electric charge having the polarity reverse to the toner charge is injected into the toner at a position just after the fixing nip portion N, so that the polarity of the toner is reversed. Therefore, the toner is in a condition apt to adhere to the fixing roller 40 or the fixing film 63, which might cause toner contamination.
In the case of a low cost heat fixing apparatus that is not provided with particular cleaning means on the surface of the fixing roller 40 or the fixing film 63, the toner contamination gradually accumulates on the fixing roller 40, the fixing film 63 or the pressure roller 50 that is in contact with them, as the heat fixing apparatus performs the heat fixing process on a large number of recording materials. Then, the accumulated toner is sometimes discharged onto the recording material (this phenomenon will be referred to as blobs hereinafter) to cause an image error.